It is desirable to increase the energy density of lithium ion batteries. This is generally achieved by increasing the operating voltage and/or by increasing the capacity. For example, LiCoO2-based batteries are usually charged to about 4.2 V and the capacity of LiCoO2 is less than 150 mAh/g at this voltage. On the other hand, LiNiO2-based batteries are charged to still lower voltage.
Increasing the charging voltage of LiCoO2-based batteries to 4.3, 4.4 or 4.5 V versus Li/Li+ will significantly increase the reversible capacity of LiCoO2 to about 155, 175 or 195 mAh/g. LiNiO2-based batteries can achieve a similar capacity at lower voltage. Increasing the charging voltage, however, causes some problems such as the excessive evolution of gas, excessive build-up of cell resistance, decomposition of electrolyte, etc. These problems become more serious, especially during cycling or storage at elevated temperatures.
Many publications and patents have suggested coating approaches to protect the surface of cathode active materials (active material of positive electrode) and the improvement of high voltage cycling stability by this method was confirmed. However, in many cases, the observed improvement did not result from an effective coating but from a shorter air exposure of the coated material after heat treatment (Z. Chen, J. R. Dahn, Electrochemical and solid state letters 7 (1) A11-A14). Application of the described coated material to commercial batteries did not sufficiently solve the high voltage and elevated temperature problems.
Although it was confirmed that avoiding air exposure for a long time can improve material properties to some degree, especially during cycling at room temperature and in small cells, it is generally difficult to implement shorter air exposure during large scale production, and relevant properties are not sufficiently improved under more severe conditions such as high voltage and elevated temperature.
In order to overcome these problems, several patent applications suggest addition of LiF; for example, U.S. Publication No. 2004-91780 A1 discloses the addition of LiF and LiOH to mixed hydroxides prior to a solid state reaction, and U.S. Publication No. 2002-14222 A1 discloses doping of halogen to high crystalline LiCoO2. However, the film-forming property of LiF is generally poor, because LiF is not a lithium acceptor and does not contain a dopant cation, thus a chemical reaction between LiF and the surface of the particle is absent. As such, the surface is less protected, whereby the high temperature/high voltage properties cannot be sufficiently improved, or a large amount of LiF needs to be added, which however decreases the capacity.
As an alternative approach, U.S. Publication No. 2003-104279 A1 discloses the addition of MgF2 as a dopant compound to LiCoO2. MgF2 might be suitable for spinel or Li—Ni—Mn—Co-based materials, but it is not recommended in the case of LiCoO2. Firstly, Mg2+ is a less suitable dopant for LiCoO2 than Al3+. While a solid state solution represented as LiCo1-xAlxO2, i.e., a solid state solution of LiCoO2 and Al, is well known, it has been not confirmed whether a solid state solution can be obtained from LiCoO2 and Mg. If the solid state solution of LiCoO2 and Mg could be made to exist, it would result in a defective structure and deviate from LiIMIIIO2 representing an ideal composition of solid state solution. Secondly, the melting point of MgF2 is very high, thus the reaction kinetics at the surface is low and the tendency to form a thin protective film is reduced.
U.S. Pat. No. 6,613,479 discloses the doping of fluorine to layered LiMnO2, including doped LiMnO2, wherein different classes of materials are dealt with and Mn is trivalent. However, the materials in the above patent are prepared in inert gas at low temperature and also generally have poor crystallinity. As will be illustrated later, the powdered lithium transition metal oxide of the present invention can be prepared in air and is also stable in air and is crystalline, and furthermore, the manganese would be tetravalent.
In addition, many prior arts show various coatings such as oxides, phosphates, borates, glassy phases etc., surrounding particles of cathode active material, and they are generally made by coating techniques such as dipping, sol-gel, slurries containing sub-micrometer particles, dry coating, etc. For example, in U.S. Pat. No. 6,372,385, cathode powders are dipped into a gel. However, these prior arts fail to provide lithium transition metal oxide having thermodynamically and mechanically stable properties.